/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/DynamicMeshOverlay.h
Create a new copy of ElementID, and update connected triangles in the TrianglesToUpdate array to reference the copy of ElementID where they used to reference ElementID. The new element will have the given parent vertex ID. Deletes any elements that are no longer used after the triangles are changed.
the ID of the newly created element
int SplitElementWithNewParent ( int ElementID, int SplitParentVertexID, const [TArrayView](API\Runtime\Core\Containers\TArrayView)< const int > & TrianglesToUpdate )